1. Field of the Invention
This invention relates to a method for installing an offshore jack-up drilling structure, more particularly to a method for installing and operating an offshore jack-up drilling rig at a desired offshore location, and to countermeasure for minimizing the occurence of punch-through accidents during preload opration.
2. Description of the Prior Art
In general, an offshore jack-up structure or rig consists of a buoyant platform and a plurality of movable legs and such type has been widely used for offshore operations, such as drilling and completing under water oil and gas wells. In the condition in which the movable legs are fully raised up, the buoyant platform is towed to a desired location, then the legs are lowered by a jacking system so that the platform is supported on the sea floor. Finally the platform is raised above the surface of a body of water for operations. For the purpose of raising or lowering the legs of the platform, a "rack and pinion type" jacking system is widely used. The jacking system of this type includes at least one elongated rack which is mounted vertically on the exterior side surface of the upright legs and extends substantially through the entire length of the same, and a plurality of cooperating pinions engaged with each of the racks. Each of the pinions is driven through a series of reduction gears by means of a respective electric motor.
It is required to position the rig at a desired offshore location and then the raised legs with a footing are lowered down to contact with the sea floor. After pumping water into water tanks provided in the buoyant platform so as to apply preload to the jack-up rig for driving the footing of the leg into a substantial depth in the ground beneath the sea floor, the platform is raised to a suitable elevation above the surface of a body of water.
During this preload operation it is reported that a number of accidents have been occured due to the rig inclination by unexpected penetration, viz.punch-through, of the legs into the sea floor. Particularly, the recent trend toward heavier jack-up rigs being built to cope with deeper and harsh seas of operation calls for effective countermeasures against such sudden penetration (punch-through) of jack-up rig legs.
When a pre-installed production assembly, hereafter referred to as a "target structure", is in want of repair by using the rig, old footprints of the legs would be left on the sea floor. If the legs are lowered down at the vicinity of the old footprints again, an unexpected trouble will be happened.
Consequently, it is necessary to carry on investigation of configuration of the ground, the nature of the soil of the ground beneath the sea floor or the depth of water.
In addition to the above, the following data will be requested for installing and operating the rig:
(1) the draft of the platform;
(2) the depth of water in taking consideration of a tide level or an average wave height into account;
(3) the distance between the lower end of the footing and a refrence level of the surface of a body of water during the installation of the rig;
(4) the distance between the lower end of the footing driven or penetrated into the ground beneath the sea floor and a reference level of the surface of a body of water;
(5) the rate of subsidence of the leg per unit time of about 30 seconds;
(6) an angle of inclination of the surface of the sea floor;
(7) an angle of inclination of the platform supported by the legs on the sea floor;
(8) operation of jacking mechanisms;
(9) a height of the leg from the deck of the platform;
(10) the direction and speed of the wind;
(11) the direction of the tide;
(12) the height of the wave;
(13) weight with load applied to the rig by pumping water into water tanks in the platform;
(14) load to be applied to each of the legs;
(15) the length of a cantilever extending from the platform; and
(16) the distance between the surface of the sea floor and the lower end of the leg or footing to be penetrated into the ground beneath the sea floor.